The present invention relates to a silacyclopentadiene derivative. In more detail, the invention relates to a silacyclopentadiene derivative and an electroluminescent (EL) element using said derivative.
Recently, organic EL elements have been noticed as candidates for plain displays having such a high luminance which has never been obtained, so that studies and developments thereof have been activated. The organic EL element has such a structure that an organic luminescent layer being sandwiched by two electrodes, and it emits luminescence by recombination of holes injected from an anode and electrons injected from a cathode in a luminescent layer. The organic materials used for the said organic EL elements include low molecular weight materials and polymer materials, both of which can make EL elements with high luminance. There are two types in such organic EL elements. One type is an element having a fluorescent dye doped in an electron-transporting layer which has been published by C. W. Tang and et al (J. Appl. Phys.), 65, 3610 (1989)), and the other type is an element having a fluorescent dye singly used (such as an element described in Jpn. J. Appl. Phys., 27, L269 (1988)). In the latter element, it was shown that a luminous efficiency was improved in the case of a fluorescent dye being laminated with a hole-carrying layer for only carrying holes, which are one of electric charges, and/or with an electron-carrying layer for carrying only electron. Although many various materials such as triphenylamine derivatives are known as hole-transporting materials used for organic EL elements, there are a few electron-transporting. Furthermore, the known electron-transporting materials have lower charge carrier transporting abilities than the known hole-transporting materials such as N,N'-diphenyl-N,N'-di(3-methylphenyl)-4,4'-diarninobiphenyl (TPD), and in the case of being used for organic EL elements, ability thereof can not cause sufficient elemental characteristics because of limitation by the used electron-transporting materials.
As examples of such electron-transporting materials, metal complexes of oxine derivatives (Denshi Joho Tsushin Gakkai Gijutsu Kenlyu Hokoku, 92(311), 43(1992)) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) etc. have been known. The former can operate organic EL elements at a relatively low voltage, but insufficiently, and it is difficult to obtain blue luminance due to the own green luminance. As examples of the latter used as electron-transporting layers, there have been organic EL elements described in the above-mentioned Jpn. J. Appl. Phys., 27, L269 (1988). It was, however, pointed out that stability of the thin film is poor due to a tendency of crystallization about the said described organic EL elements, and then compounds having multiple oxadiazole rings have been developed (such as those described in Nippon Kagakukai-shi, 11, 1540 (1991), Toku-Kai-Hei 6-145658, Toku-Kai-Hei 6-92947, Toku-Kai-Hei 5-152072, Toku-Kai-Hei 5-202011, and Toku-Kai-Hei 6-136359 etc.). They however have not sufficient properties for practical uses such as high operative voltage. As an another compound type, quinoxaline derivatives have been reported (Toku-Kai-Hei 6-207169). Although stabilities of the thin films are improved by dimerizing quinoxaline to increase their molecular weights, they are insufficient for practical uses because of high operative voltage.
As characteristics of the electron-transporting materials used for the said organic EL elements, superiority in electron-carrying ability is necessary first.
On one hand, there have been mentioned those disclosed in Toku-Kai-Hei 7-179477 official gazette as recent reported examples of the silacyclopentadiene derivatives, but they related to reactive intermediates with intention of application to .pi.-electron conjugated type organic polymers, not to the organic EL elements according to this invention. Furthermore, examples of copolymers with thiophene have been disclosed in Toku-Kai-Hei 6-166746 official gazette, but these compounds are unsuitable as electron-transporting materials for organic EL elements because of long absorption wavelength and luminous wavelength.
Furthermore, there were examples of silane derivatives utilized for organic EL elements in Toku-Kai-Hei 6-325871 official gazette, but those having silacyclopentadiene rings were not shown amongst the disclosed organic silicone compounds therein, and also the said derivatives are low in electron-carrying property, there being not described any usefulness as electron-transporting materials.
Furthermore, there were examples of silane derivatives utilized for organic EL elements in Toku-Kai-Hei 5-343184, Toku-Toku-Kai-Hei 6-124784, Toku-Kai-Hei 6-234968, Toku-Kai-Hei 6-293778 and Toku-Kai-Hei 7-11244 official gazettes, but silacyclopentadiene rings were not included in the disclosed organic silane compounds therein, and also there has not any description about an electron-transporting property and the practical use examples were only as interfacial layers for improvement in adhesively between hole-transporting materials or luminescent layers and anodes, so that there was no description as electron-transporting materials.
The present inventors had studied zealously in order to solve these problems and to find organic EL elements with low voltage and high luminous efficiency. As a result, it was found that silacyclopentadiene derivatives to be used for organic EL elements can solve the above-mentioned problems to complete the present invention.
As clear from the above description, an object of the invention is to provide organic EL elements by which high luminant emission can be obtained at low voltage.